High Temperature Gas Control for Automotive Test Cell

Environmental-related testing is of particular importance to the automotive sector. However, emissions from gas or diesel engines can be characterized, regulated, or controlled only if they are accurately measured.

The following case study describes the use of advanced gas control valve technology by an automotive test system manufacturer. This application required a robust device for modulating high-temperature exhaust gas as part of a backpressure measurement solution in an engine test cell.

Background

Test cell equipment is commonly found in a wide range of automotive research, development and production environments. For example, test cells are utilized to develop, characterize and test new engine designs. The test apparatus allows engine operation over varying performance criteria and offers a wide range of measurements, including: coolant flow for heat balance studies, oil flow to ensure proper lubrication, fuel and intake air flow to optimize combustion efficiency, and exhaust gas flow to assure the engine and catalytic converter system comply with clean air guidelines.

During the development process for new vehicles, specifically work involving combustion, torque drive train and auxiliary components, Original Equipment Manufacturers (OEMs) use test cells to verify 100 percent certification of performance and reliability over a part’s full range of inputs. Each component must pass rigorous validation procedures for design acceptance.

Uses for a Test Cell

There is a continuous demand for precision automotive testing solutions for research, development and end-of-line testing, including media supply and control, automation and measuring equipment. These can be single, stand-alone test benches or entire test facilities.

An automated engine test stand typically houses various sensors or transducers, data acquisition devices and actuators to control the engine state. The sensors measure physical variables of interest, such as:

• Crankshaft torque and angular velocity
• Intake air and fuel consumption rates
• Air-fuel ratio for the intake mixture
• Environmental pollutant concentrations in exhaust gas
• Different configurations of hydrocarbons and nitrogen oxides, sulfur dioxide, and particulate matter
• Temperatures and gas pressures at several locations on the engine body
• Atmospheric conditions such as temperature, pressure and humidity

Backpressure Application

Suppliers of automotive testing equipment provide highly complex systems to examine the exhaust gases of production vehicles and prototypes under all reproducible climatic conditions. These systems may feature a chassis dynamometer, precision fuel metering, airflow simulation, combustion air and CVS exhaust dilution bag sampling, fuel tank temperature control, and other sophisticated capabilities.

Pressure is an important part of engine testing, emissions monitoring and automotive component testing. That’s why wherever there are engines being tested, there are likely  transducers and valves measuring and controlling everything from air intake pressures to exhaust backpressures.

A slight pressure in an engine’s exhaust system is normal. However, excessive exhaust backpressure seriously affects engine operation. It may cause an increase in the air box pressure with a resultant loss in the efficiency of the blower. This means less air for scavenging, which results in poor combustion and higher temperatures.

Choosing the Right Solution

Experience has shown that measuring liquids and gases used in automotive test cells demands superior instrument performance. For example, a test equipment manufacturer building an engine testing apparatus for an OEM sought a solution for modulating high-temperature (850° F) exhaust gas in their system. The measurement process required precise backpressure control, including a control valve capable of modulating to maintain an inlet pressure of 300 psig (the outlet pressure was atmospheric). The required flow rate, given P1 and P2, called for a Cv of 21.

This application presented three primary challenges:

• The fluid temperature was beyond the capabilities of a normal valve and packing.
• The psig shut-off pressure was very high.
• Accurate and reliable pressure control was necessary for a successful application.

The test equipment manufacturer consulted with SW Controls, Inc. (Farmington Hills, Michigan) for assistance in selecting an appropriate control valve for their application. SW Controls is one of the Midwest’s largest process control representatives and distributor organizations, stocking a large inventory of control valves, flow meters, transmitters, controllers, and recorders.

After considering alternative valve technologies, the Badger Meter Research Control^® Valve (RCV) Model 9000 valve was selected for the engine test cell. The Model 9000 is an ANSI Class 300 valve with a bolted bonnet and post-guided inner-valve. It is designed for modulating control of liquids and vapors in environments where compact size, coupled with the ability to withstand high temperature and pressure, are essential.

The RCV valve was well-suited to meet the end user’s requirements due to its multiple trim sizes and actuator bench ranges. The device’s optional graphite packing and unique extended bonnet were other important features, since unlike common Teflon^® packing, the graphite packing will not melt under extreme temperatures.

The valve employed in this application featured a two-inch, 316 stainless steel wafer body and was configured with a size 35 air-to-open/fail-closed actuator with six springs, providing a 6…30 psig bench range. It utilized a 12-psi preload and was installed in the flow-to-open direction. The device trim had a Cv of 21 with a linear characteristic.

The valve actuator’s pre-formed diaphragm and multi-springs ensure extremely linear travel versus input signal performance. Plus, its single “O” ring and Nylatron guide bushing provide minimum hysteresis. Users can adjust the spring preload to suit their specific closing force requirements. The actuator also includes adjustable travel stops. Featuring powder-coated steel construction, this design offers added protection against abuse and corrosion.

The RCV control valve was mated to an electro-pneumatic positioner to position the valve based on a 4-20 mA control signal. The positioner delivers fully automatic determination of the control parameters and adaptation to the final control element.

Project Results

The test equipment manufacturer worked closely with SW Controls to specify the right gas measurement and control solution for the automotive test cell. The project was a success as it met the following key criteria:

• The high process temperature led to the selection of 316 SS valve body material, as well as a specialized, 10-inch extended bonnet and graphite packing.
• The high shut-off pressure required the use of a 6…30 psig bench range with 12 psig preload. The 300 psig process inlet pressure multiplied through the large Cv creates a strong upward thrust, which tries to lift the plug off the seat.Compressing the actuator springs with a 12 psig preload opposes the upward thrust and successfully holds the valve closed in its fail-safe condition.
• The use of a high-accuracy electro-pneumatic positioner strokes the valve from closed to full open as the control signal ramps from 4 to 20 mA. The valve’s graphite packing, necessitated by high process temperature, adds hysteresis; however, the positioner overcomes this condition and provides smooth control, taking advantage of the valve’s  50:1 rangeability.

Conclusion

The success of this application can be attributed to the broad product offering of Badger Meter, coupled with the consultative capabilities and automotive industry experience provided by SW Controls.

Badger Meter leverages more than a century of flow measurement expertise and advanced, yet proven technology to optimize the most demanding applications. It offers an extensive portfolio of products, services and solutions to measure and control water, air, steam, oil, and other liquids and gases.

The RCV line of control valves is constantly developing and evolving in order to satisfy ever-changing application demands in the field. Featuring a wide breadth of configurable and compatible options, RCV valves can be designed to perform strongly in a wide variety of demanding flow control applications.

SW Controls maintains a staff of dedicated application and sales engineers, inside support personnel and factory trained service technicians to assist customers with their requirements.

To date, the automotive test system has been operating precisely and the Badger Meter RCV gas control solution is exceeding the end user’s expectations in a challenging application environment.